OBJECTIVE: It has been suggested that impaired heart rate variability (HRV) may be an early sign of Parkinson\'s disease (PD). The aim of this study was to determine whether HRV can be employed in order to differentiate between dementia with Lewy bodies (DLB) and Alzheimer\'s disease (AD). METHODS: We examined HRV in 30 probable DLB patients (16 men and 14 women; mean age, 79.9 years; SD, 4.7 years), and 30 probable AD patients (15 men and 15 women; mean age, 79.8 years; SD, 5.6 years), compared with that in 20 age- and sex-matched controls. Subjects with other causes of impaired HRV were excluded. HRV was determined using the RR intervals of a 5-min electrocardiogram recording. Measurements of beat-to-beat RR variability, including time domains [(RR-standard deviation (SDNN), percentage of consecutive RR intervals differing by more than 50 msec (pNN50), and root mean square difference of successive RR intervals (RMSSD)), and frequency domains [very low- (VLF), low- (LF), and high-frequency (HF) components, and total spectral power (Total power)], were assessed retrospectively. The association between these HRV parameters and cardiac iodine-123 metaiodobenzylguanidine ((123)I-MIBG) scintigraphy were investigated in 22 probable DLB patients. RESULTS: DLB group showed significant decreases compared to AD group in almost all HRV parameters including SDNN, pNN50, RMSSD, VLF, LF, HF, and Total power. Among these, SDNN, VLF, and Total power were correlated with the (123)I-MIBG delayed heart to mediastinum ratio. CONCLUSION: Impaired HRV was detected in patients with probable DLB. Non-invasive and routine electrocardiogram may have potential in differentiating DLB from AD.